The invention relates to an electromechanically and hydraulically actuatable motor-vehicle brake with an actuator assembly comprising:                a housing,        an actuator, displaceable relative to the housing, for hydraulic or electromechanical displacement of a brake pad,        a motor drive,        a displacement mechanism arranged between the motor drive and the displaceable actuator,        a gear arrangement assigned to the displacement mechanism, and        a separate self-locking device which is designed to block the displacement mechanism as needed,        wherein the displacement mechanism exhibits a ball screw with a spindle and a nut, wherein optionally one component from spindle and nut is capable of being driven in rotation and the respective other component from spindle and nut is capable of being linearly displaced within the housing for the purpose of displacing the actuator by driving in rotation the one component from spindle and nut, and wherein the gear arrangement exhibits at least two gear stages.        
For some time, motor-vehicle brakes have been employed in which, in the case of a normal service braking during the travel of the vehicle, the brake pads are displaced hydraulically in the usual way, but in which for the purpose of activating a parking-brake function an electromechanical displacement of the brake pads or at least an electromechanically triggered blocking of the brake pads in a braking position also occurs. Motor-vehicle brakes of such a type offer the advantage that the parking-brake function can be activated or released more conveniently by simple actuating of an activating switch.
Electromechanically and hydraulically actuatable motor-vehicle brakes of such a type are known from the state of the art.
For instance, document WO 2008/037738 A1 describes a motor-vehicle brake that can be actuated both hydraulically and electromechanically. In a normal operating situation—that is to say, during the travel of the motor vehicle—this motor-vehicle brake is actuated hydraulically in conventional manner. For the purpose of activating a parking brake, however, the electromechanical actuation function is activated. In this case an electric motor is driven which drives a spindle/nut arrangement via a displacement mechanism with a gear arrangement. The gear arrangement is designed to be self-locking with a worm gear, in order to prevent the parking-brake action from diminishing when the parking brake has been activated. But the self-locking has the disadvantage that only very low efficiencies can be attained, so that the components, in particular the electric motor, have to be designed to be relatively strong and have a high current consumption. It is considered that the overall efficiency of the system is made up of a product of the individual efficiencies of the components. For instance, this overall efficiency results from the product of the efficiency of the engine, the efficiency of the downstream gear arrangement and of the spindle/nut arrangement. In gear arrangements with self-locking effect, overall efficiencies can accordingly only be attained within the range of 30% and less.
Document DE 10 2012 208 294 A1 describes a vehicle brake in which a separate self-locking device is arranged directly on the spindle/nut arrangement. In particular, reference is made to FIG. 8 of this document. Therein it is shown that the self-locking device designated as coupling device 41 is arranged between the rolling-element ramp transmission and the brake piston. Such a direct spatial assignment of the self-locking device to the nut/spindle arrangement of a ball screw has the disadvantage that the reaction forces arising in the course of the activation of the brake and in the course of the maintenance of a (parking) braking state have to be borne in their full magnitude by the self-locking device. The self-locking device has to be constructed to be correspondingly sturdy, having the consequence that it takes up considerable construction space. This has the result that the brake has to be dimensioned to be relatively large, particularly in the axial direction with respect to the threaded spindle.
Furthermore, document DE 10 2011 102 860 A1 presents a vehicle brake which has been improved in comparison with this state of the art, in which the self-locking device is in fact arranged close to the nut/spindle arrangement but is integrated into the gear arrangement, so that construction space can be saved in the axial direction with respect to the threaded spindle. This brake can be configured to be more compact.
Document DE 196 54 729 A1 discloses a motor-vehicle brake having the features of the precharacterising portion of Claim 1. Further state of the art is to be found in documents U.S. Pat. No. 5,829,845 A and DE 103 49 078 A1.